Apparatus for developing electrostatic latent image

ABSTRACT

An apparatus for developing an electrostatic latent image on an insulating layer having an electroconductive backing layer by spacing the layer from a developing electrode in a developing liquid where either or both the developing electrode and the electroconductive backing layer are grounded via resistance in the range of 106 - 1012 Omega .

United States Patent 11 1 6/1967 Oliphant et a]. 118/637 Sato July 31,1973' [5 APPARATUS FOR DEVELOPING 3,551,146 12/1970 Gundlach 118/637ELECTROSTATIC LATENT IMAGE 3,405,683 IO/1968 .Ions et al 118/6373,577,259 5/1971 Sato et al 118/637 Inventor: Mmmlchl a Asakl-shl.Sanama, 3,409,358 11/1968 Fauser 118/637 Japan 3,599,605 8/1971 Ralstonet al.... 118/637 3,368,894 2 1968 Matkan et al.... 96/1 [73] Asslgnee-3,554,161 1/1971 Blanchette 118/8 j Kanagawa, Japan [-22] 1 Filed: *5'1970 "Primary Examiner-+Mervin Stein [21] AppL- No: 67,473 I AssistantExaminer-Leo Millstein v 1 Attorney'-'Addams & Ferguson [30] ForeignApplication Priority Data Aug. 27, 1969 Japan 44 67795 [57] ABSTRACT[52] Cl 118/637 117/37 gg f d An apparatus for developing anelectrostatic latent image on an insulating layer having anelectroconduc- 2; 'igbg z q tive backing layer by spacing the-layer froma develop- 1 0 l ing electrode in a developing liquid where either or I37 355/10 both the developing electrode and the electroconductive backinla er are rounded via resistance in the [56] References Cited range of os 12 g UNITED STATES PATENTS 3,328,193 12 Claims, 14 Drawing Figures i II v 4 Pmmtnww 3.749.059

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PATENIEUJUUI I915 I 3.749.059

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sum 3 or 4 Z ll/lj/ll/ INVENTOR J mo APPARATUS FOR DEVELOPINGELECTROSTATIC LATENT IMAGE BRIEF EXPLANATION OF THE DRAWING tribution ofelectriccharge onthe surfaceof theelectrophotographic materialillustrated in FIG. 1.; FIG. 3

is adiagram showing thedistribution of electric field in the; directionperpendicularto the surfaceofsheetzas derived from:theqdistributionofel'ectric charge illustrated in FIG. 2 in the; absence. of adeveloping electrode; FIG. 4 isa-diagram showingthedistribution ofelectric field as derived' when a. developing electrode is groundeddirectly. and-.- when itis. grounded via agroundingresistance; FIG. 5 isa side elevation in longitudinal section offthe developing. electrodeand the sheet-shaped" electrophotographic material grounded via thegrounding resistance; FIG& 6. is adiagram' showing the, distribution of1 electric: charge derived where there is residual electriccharge onthe: sheetsurface; FIGS; 7'and8- arediagramsshowing the distribution ofelectric field" as derived where the developing electrode. is grounded:directly and it is grounded -via'thegrounding resistancerespectively inthe case 'ofthe 'diagram'of FIG. 6;,FIG. 9,.throughFIG; ll and FlG.l4-are side elevations in longitudinal-section of devices employed for:puttingthis invention to working; and FIG. 12 and FIG. 1-3 are sideelevations-in longitudinal section of the structure of bearingoftheroller illustrated. in FIG. 11'.

DETAILED DESCRIPTION OF THE INVENTION This invention-relatesto anapparatus for developing electrostatic latent image; More specifically,this invention relates. to. a liquid developingyapparatus so improved asto prevent theoccurrence. of'fogging (a phenornenonv which obscures the.background to uniform density) due to residual electric charge andfiofstreaks in the image.

Photoconductive insulating layers used'in the fieldof electrophotographycommonlyprocess more or less residual electric potential. Whenaphotoconductive insulating layer-is electrically chargeduniformly andsubsequently exposed to. light throughan image, theelectric chargeisneutralized in thearea where the layer has been exposed to light but thecharge is retainedintact on the photoconductive insulating: layer in thearea where the layer has escaped exposure to' light. Theelectriccharge-to be neutralized increases with the in-- tensity oflight to which the layer is exposeda'lhus, an electrostatic latent imageconformingv to the. image which is represented in terms of lightintensity is producedonthephotoconductiveinsulating layer.ln'actualit-y, however, thedegreeof neutralization of electricchargestops growing proportionally with the rise of lightintensity whenthe light intensity exceeds a certain level, beyond: which the: degreeof neutralization growth is lowered." When the .exposureto light throughtheimageisstopped before completeneutralization of electric charge whilethe intensityof-Iight for illumination 'isatits maximum ,':electriccharge is suffered to re* main even in areas where .electric charge.wouldotherwise be neutrali'zed completely. Consequently, there isobservedthe so called-phenomenon of residualtelectric potential. Whenthe exposure to light of high-intensity is continued until perfectelimination of residual electric potential, electric charge isneutralized excessively in areas where electric charge. should not beneutralized beyond a certain level. Thus, it becomes no longer possibleto produce an electrostatic latent image faithful to the original image,namely, properly proportional to the intensity of light used forexposure. In reality, therefore, there is followed a practice by whichexposure is effected to such extent astoallow residual electric chargeto occur to some extent and subsequently eliminate fogging due toresidual electric charge by a suitable means. If the residual electriccharge is'devel-. oped faithfully, the background portion of image isfoggeduniformly to produce a very unsightly image.

Heretofore, there has been proposed a practice ofapplying DC voltage toa developing electrode from an external source so as to give rise toelectric potential equalling to the residual electric charge andconsequently prevent the charge-from producing fogging.

The conventional method and apparatus has been found defective in thattoner particles are accumulated on'thedevelopingelectrode and tend tocomeinto con,- tact withthe electrostatic latent image and, as a result,smear the image. The loss of toner particles because 0 such accumulationhas been heavy.

This invention relates to an apparatusfor developing an image free fromfogging due to residualcharge and faithful-to the original withoutrequiring any external power source for the neutralization of residualpoten tial.

One moredefect in liquid development is occurrence of streaks indeveloped image. Next steps are usually involved in makingaprint byelectrophotography; neg.- atively charging an electrophotographicmaterial, c.g., a paper bonded a photoconductive layer consistingofzinc-oxide pigment in a resin binder, exposing it to form a latentelectrostatic image, and developingithe latent image with positivelycharged fine particles dispersed in insulating liquid. In thisdeveloping process, the flow of liquid developer produces streaks. Thestreaks are conspicuous inthe'case the flow of liquid developer is fastor the potential of the latent image is high. This phenomenon easilyoccurs when the liquiddeveloper-is' poured into the space between thesurface bearing a latent image and the developing electrode held close"to the surface.

In a developing apparatus wherein an electrostatic latent image formedon an insulating layer of an electrophotographic material is disposed soas to-confront a developing electrode with a minute opening and via adevelopingliquid havingfinely divided charged particles suspended in anelectrically insulating liquid, this invention is characterized byconferringupon either or both of the developing electrode and theelectroconductive layer of electrophotographic material such magnitudeof grounding resistance'as to permit prevention of excessive depositionof the aforementioned charged particles-in the highlight zone ofelectrostatic latent image (the zone where the surface potential issmallest).

When the effect of developing electrode is main- I tained at anextremely high level throughout the period of development, even thesmallest amount of residual potentialdue toresidual charge is developedfaithfully to produce fogging. Therefore, the spirit of this inventionresides 'in' conferring an adequate magnitude-off grounding resistanceupon either or both of the devel- 3 oping electrode andelectroconductive layers of electrophotographic material, whereby tonersare prevented from depositing in the zone of the least charge densitywithin the electrostatic latent image (namely,

further detail by referring to the drawing.

FIGQl shows a condition under which electric charge exists uniformlyonly in the right half portion of surface on the electrophotographicsheet 10. Numeral 11 denotes a'photoconductive insulating layer whichmay be made of any known substance. Examples are amorphous selenium andphotoconductive zinc oxide powder blended respectively with aninsulating resin. Numeral l2 denotes an electroconductive base. Thematerials usable therefore include metal plate, plastic film treated soas to acquire electroconductivity, and paper. In the course ofdevelopment, the base 12 is usually grounded. The latent image of FIG. 1represents a case wherein positive charge is distributed uniformly inthe right half portion of the upper side of the photoconductiveinsulating layer 11 while negative charge of an equal magnitude existsin the half portion of the lower boundary.

FIG. 2 is a graph illustrating the distribution of positive charge onthe layer 11 as shown in FIG. 1. In the graph, the origin represents theboundary between the zone containing electric charge and the zone devoidof electric charge as shown in FIG. I. It is assumed that the electriccharge is uniformly distributed at a density of 0'0.

.FIG. 3 is a graph illustrating the composition of electric fieldsdistributed perpendicularly to the plane of latent image as formed, inthe absence of a developing electrode, within the electric field closeto the latent image which is produced by distribution of electric chargeas shown in FIG. 2. The distribution of electric charge shown in FIG. 2has an entirely different shape from the distribution of electric fieldshown in FIG. 3. It is universally known that the phenomenon of edgeeffect results consequently. What should be noted at this point is thefact that even at the position satisfying 0' 0 in the diagram of FIG. 2,a negative electric field (with the electric field lying outward fromthe plane of latent image taken as a positive field) occurs in FIG. 3.This shall be referred to hereinafter as opposite electric field.

FIG. 4 is a diagram illustrating the distribution of electric fieldscorresponding to those of FIG. 3 where a developing electrode isdisposed to confront the surface containing the latent image at a fixeddistance. In FIG. 4, the curve A represents a case wherein the groundingresistance of developing electrode (hereina after expressed as R") isvery small (R 0), while the curve B represents a case wherein thegrounding resistance is very large (R 0, for example). When R becomes assmall as this, a fair agreement is shown between distribution ofelectric charge and that of electric field. Yet, there still remains avery minor magnitude of opposite electric field. Particularly, theopposite electric field in the neighborhood of edge (the boundary ofdistribution of electric charge, namely, the origin of the graph of FIG.4) is much larger than that at a distance. The opposite electric fieldbecomes muchnitude of grounding resistance R fixed at an extremely highlevel. Negative charge is induced through electrostatic induction in theareaof the developing electrode confronting the positive charge oflatent image. In the other portion of the developing electrode, positivecharge substantially equivalent to the induced negative charge isinduced. Because of a large value of R, the positive charge is notimmediately discharged to the ground. This positive charge can moveabout with considerable freedom within the developing electrode. Of thenegative charge distributed as opposite charge of latent image in theboundary of the electroconductive layer 12, the magnitude of negativecharge equivalent to the negative charge induced on the developingelectrode acquires freedom of mobility and eventually confronts thepositive charge induced on the developing electrode where there existsno electric charge of latent image, as shown in FIG. 5. Consequently, anopposite electric field is formed at this portion and properties asrepresented by the curve B of FIG. 4 are believed to be acquired.

, FIG. 6 illustrates the distribution of electric charge as obtainedwhere there exists a residual charge 0- This means that the electriccharge a 0 remains where the magnitude of electric charge shoulddesirably become 0-,, 0. The distribution of electric charge shown inFIG. 2 represents an ideal case, while the distribution of electriccharge which is actually obtained by charging and exposure to light isas shown in FIG. 6.

FIG. 7 is a diagram which is contrasted to that of the distribution ofelectric charge in FIG. 6 and which corresponds to FIG. 4. The diagramrepresents a distribution obtained when the distance between thedeveloping electrode and the latent image surface is fixed tocomparatively small and the value of R is very small. As is evident fromFIG. 7, the electric field at the portion where the residual charge 0',exists constitutes a positive field except in the neighborhood of theboundary of charge distribution. In the vicinity of boundary, the edgeeffect persists to induce negative electric field. When the distancebetween the developing electrode and the surface of latent image becomesvery small (to several 10s of u, for example), this edge effect becomesgreatly decreased. When a liquid developer (having a microfine chargedparticles or toner particles with negative charge suspended in aninsulative liquid) is supplied between the opening between thelatentimage surface and the developing electrode while the electricfield is distributed as shown in FIG. 7, the toner particles aredeposited in the portion of positive field. The consequence is thatthere occurs fogging due to residual electric charge.

The reason why the streaks shall occur has been,

guessed as follows. When the liquid developer is applied to the surfaceof the latent image, for example,

applied to the left direction in FIG. 2, the toner particles, havingnegative charge in this example, are attracted to the positively chargedright half area. Some 4.. urveA- 5, P r c s re car ied down rdr mhe-area, and. attach even to the noeharged area. 'l hisis, significantin the casethe potential of charged area is the higher. In general, thecounter'electrical field (repulsion field) which prevents the attachmentof toner; particles appearson the no charged area. When the, counterfield srweak he tonerpat iolesdoposit ontthe no ch ge 'otoai hough heenulsion fieldi s rong. in. the. ab-- n o; of deve pi g elec ode s howmin FIG. s Presen s: of e ping le tr de. mak the.-r p ..lsion oldz alsnslat r c el s ronga sho n in-ElG.

fi sg ho; troak a-* I 1 RIG. 8;; is-a diagram. sim-ilar to that-of FlGj.7"which corresponds to-FIG. 4.: Thediagram representsa-case.

8; hev lueotRx o arge-.-(cot, espom ingtolieou e.

Bi l 161 t the ppos te. leotriofield by and. heP s t v lo t i fihargeby.0, oanoele oh witharesultthat o positivefield;dev op :evenhe p tio there l istsr sidualcharge e e oner par le positsat th sportion- It h le-of ra opt a a-rhislt ore hr gho t thet ge of e pr on t there occurshoodgel ffeot nd-t ere c nseql en y n uostho oallodihalm a phenomenon oflajcle. of toner deposition in the zoneof the low. charge density closeto theboundary between thehighcharge densityland the low, chargeidensity, Therefore, it must be. avoided .to. keepthe value of R? at a.high level,

ou ho t the.ontir meri dof ev l pm -1 h fore it becomes necessary ,to,lower thevalue-of B after lapseof ;a proper time. The Jengthof this timeis not fixed but i varied by the or properties of electrophotographicmaterialand the resistanceof-liquid developer. Generally, the. time;is..on ;theorder of several tens of econ s ut ay varyn e ange f om se eraleci mit o. v r l. m n tes tdosorint onr as b en made s. egards the ,gro-i ns o st noo en mpa ted o h dev pi g s trode, Similar;effectcan"befljobtainod by perfectly ro ndinstho eve ping .olo t odo dallowing th ectr photosr ph o ma er al. (the i ernr e by meral IZ- inFIG. l) to acquire groundingresistance. It

is naturally permissible to. have both developing elec-a de n v lectoconductiv ayer- 1 l ph g aphic a er a cquir gt unding resistanoe. Fromhe p a t alp rt fviow. f eedom, of opera ions inreased nd quip ne t. rmp' fi d y pr v grounding resistance for; the developing electrode.lnd., t,do Qfh skind an e utilized for any d f leqt opltotsrophiomaterial. I Atspecifio means of o fer ing groundin esistan eonthe electroconduc-v tive layer- 0f, electrophqtographic material,consists in noorporstina .s res stanc :l yo v onesth mlh .elec-trrronduct v yer f electrophotoattttih o "tutorial or forming theaforementioned electroconductive vvlayer ne base. Possessed of rsistance FIG. 9 is .a side elevation in cross section of one; .pret odmbodim nt for, putti g the method .of this invention ,t o .workingNurneralflo\irt FIG. ;9 denotesandlessbo f r onv y na thes eotfshape l oroph Aeeordingly, developingjelectrode;intensistashi l-mn e iali10; wihei tent i ge face he eo facin -i PP-Wa fd yi s su belt. 1 here m y,besod, flex ble,mota r el r:motalgauze belt; Plastic belt,- ubber". bet; an fabric-be t: T e ndless elt is i ri n a arqunda pai s tene s-9..,and 2i; he sheet. 10' is re o t ed on thebe t. 9 and co vey d om r tot The electroconductive layer 10 isgrounded either via the belt orindependently. The sheet'carryingplane of the belt 90 is inclined by anangle of about 5 to:.20 with reference to the horizontal plane so as tofacilitate the flow of developing liquid from rightto left.Thedeveloping, liquid 96. is stored in the container. scooped up bymeans of. a pump' which' is n'otiIlus trated; and applied to the nozzle94..At asubStantially uniform distance upward from thesheetconveyingsurfaceof the endless belt 90, there is disposedaideveloping v electrode 93 of metal plate; grounded. via thegrounding resistanceR. In onexworking example of the 1 device illustrated in FIG. 9R-isflOflJhe length of d'eveloping electrode in the direction of travelis 20 cm,

-. the space between'the developingelectrode and.the; electrostaticlatent image surface is 052mm; andthe conveying: speed of sheet isZorn/sec.

FIG. 10 illustrates another preferredv embodiment of this invention,namely, a. device suitable for developing a. web-shapedelectrophotographic material loofwhich;

is paid out of a roll. Theweb l00'retaining the electrostatic. latentimageon the. outside-iszdriven around-Jthe'" drum 10-1. The lowerportionof the. drum "isheld: in contact with the liquid developer 104;and the develop. I ing electrode 102 zhavinga curved plane is spacedwith a suitabletopeningfrorn the weband grounded-via I a resistanceR.

FIG. 11 is a sideelevation in cross section'of still an presentinvention-proves other example for which the particularly effective.

Numerals 11 191 through' 1 11 5 denote rollers which are made to rotateat a fixed speed irronesamedirection, and. numerals 112-1 throughll2-15:*are developing electroderollerswhich'areadapted .to rotate at afixedspeed in oneandsame direction while :confront ing theaforementionedrollers. .The rollers -11 l and the rollers 112 .may be held in intimatecontact or may be" separated slightly, Therollers 111 may. be .thosemade ofmetal or non-metal. lnorder thatthe rollers 1 12 may serve asdeveloping electrode, they are requiredrto beielectroconductive on thesurface or, if they are insular-"- ingon thesurface, they are requiredto be electrocon ductive in the regionclose to the surface. To be spe-wcific, metallic rollers; having the .surfacethereof ren--- deredinsulating or semi-conductive, andinsulating rollers having the surfacethereof covered with an electroconductive layer are also used. Numeralsl 13-1 through 113-6 arenozzles used for sprayingthe rollers 112 .withthe developing liquid. The developing liquid is supplied by a pump notillustrated and fed to the nozzles. The pairs of rollers 11] and rollers112 seize and: transfer the sheet 10 containing the electrostaticlatentimage on the upper surface at a fixed, speed toward rollers, '112in the example shown in FIG: 11..

v In, FIG. 12, numeral 121-. denotes a side plate made' such as of metalso as to serve as the bearingof the roll ers. 0n the side plate; thereis provided a notch-11$ for inserting the shaft 114 ,of rollers. To keepthe shaft 1 I 4 of rollers from direct contact with the side plate 121,the resisting element 122 is disposed on the surface of the notch. Aconvenient resisting element for this purpose may be prepared by mixingsilver or carbon particles into a resin or by blending electroconductiveparticles of silver or carbon into ceramic material.

FIG. 13 shows the shaft of rollers having a resisting element 13]disposed on the surface thereof instead of working on the side plate. Asthe resisting element, that which is similar to the resisting element122 of FIG. 12 may be used.

In FIG. 12 and FIG. 13, if members 122 and 131 are insulating a resistoris connected between the side plate 121 and the shaft 114.

FIG. 14 illustrates still another preferred embodiment for which thepresent invention proves effective.

In FIG. 14, some numerals as those used in FIG. 10 are used to denoteidentical items. Numeral 140 is an electroconductive liquid, which maybe water, an aqueous solution, or mercury. The liquid 140 is so selectedthat it will have a larger specific'gravity than the developing liquid104 and it will not be miscible with the liquid developer. Numeral 141denotes the electrode inserted into the liquid 140 to serve the purposeof grounding the liquid 140 via the resistance R. The electrode 141 isnot required where the liquid 140 has a high electroconductivity, but isrequired when the electroconductivity of the liquid 140 is relativelysmall (of the order of 10 for example). Where an electroconductiveliquid is used as the developing electrode (as disclosed in JapanesePatent Publication No. 44-6388,

for example), fogging tends to occur because the space between thelatent image surface and the developing electrode is very small.Accordingly, the method of this invention is extremely effective.

The magnitude of the grounding resistance to be used for the presentinvention cannot be fixed indiscriminately because it varies with theproperties of liquid developer, the properties of electrophotographicmaterial, and the nature of latent image. Where letters are reproduced,for example, R may be fairly large. In reproducing an image ofcontinuous toner, details in the highlight area cannot be reproducedwhen R is too large. The highlight area suffers from fogging if R is toosmall. It is, therefore, desirable to minimize the value of R within therange which is free from formation of fogging. Empirically, it has beenfound that R is desired to fall in the range of 10' 10 9.

Even if the developing electrode is insulated after the liquid developeris poured between the developing electrode and the electrophotographicmaterial the developing electrode is earthed through the liquiddeveloper. As the volume resistivity of the liquid developer is about10"!) cm, on the assumption that the gap between the developingelectrode and the surface of latent image is 0.] mm and that the facingarea is 100 cm, the resistance of the liquid developer is What isclaimedis:

1. Apparatus for developing an electrostatic latent image on anelectrophotographic sheet of material optionally having anelectroconductive backing layer, said apparatus comprising:

a developing electrode disposed adjacent to and spaced from saidelectrophotographic sheet of material;

means for providing a developing liquid between said developingelectrode and said electrophotographic sheet of material to develop saidelectrostatic latent image;

resistive means for connecting either or both of said developingelectrode and said electroconductive backing layer to -a referencepotential, the resistance of said resistance means being in the range of10 to 10 ohms, to prevent excessive background fogging and imagestreaking during development of said latent image.

2. Apparatus as in claim 1 wherein said reference potential is 0 volts.

3. Apparatus as in claim 1 including means for effecting relativemovement between said electrophotographic sheet of material. and saiddeveloping electrode.

. 4. Apparatus as in claim 3 including a container for storing saiddeveloping liquidand where said means for effecting relative movementbetween said electrophotographic sheet of material and said developingelectrode includes an endless belt driven around a pair of rollers, saidelectrophotographic sheet being disposed on said belt for movement withrespect to said developing electrode and said endless belt beinginclined by an angle of about 5 to 20 with respect to thehorizontalplane and where said means for providing'said developing liquid betweensaid electrophotographic sheet and said developing electrode is disposedadjacent the upper end of said endless belt to introduce said developingliquid between said electrophotographic sheet and said developingelectrode so that it flows therebetween and then to said container forsaid developing liquid which is disposed beneath the lower end of saidendless belt. I

5. Apparatus as in claim 3 including a container for said developingliquid, said developing electrode being disposed in said developingliquid and where said means for effecting said relative movement betweensaid electrophotographic sheet and said developing electrode includes arotatably driven drum having disposed around a portion thereof saidelectrophotographic sheet, said portion being partially disposed in saiddeveloping liquid and adjacent said developing electrode. 1

6. Apparatus as in claim 3 including a first plurality of drive rollersdisposed at one side of said electrophotographic sheet and where saiddeveloping electrode includes a second plurality of drive rollersrespectively disposed adjacent said first plurality of drive rollers atthe other side of said electrophotographic sheet, said first and secondplurality of drive rollers moving said electrophotographic sheettherebetween.

7. Apparatus as in claim 6 including a shaft for each of said secondplurality of rollers and a bearing for each of said shafts. Y

8. Apparatus as in claim 7 where said resistive means is disposed on thesides of said bearing and in electrical connection with said shaft andsaid bearing being connected to said reference potential;

9. Apparatus as in claim '7 where at least the outer portion of saidshaft comprises said resistance means, said shaft being in electriccontact with said bearing and said bearing being maintained at saidreference potential. I 1

10. Apparatus as in claim 3 including a container for said developingliquid and for a second liquid having a larger specific gravity than andimmiscible with said developing liquid and where said developingelectrode is disposed in said second liquid and where said means foreffecting relative movement between said electrophotographic sheet andsaid developing electrode comprises a rotatably driven drum havingdisposed around a portion thereof said electrophotographic sheet, saidportion being disposed completely in'said developing liquid andpartially in said second liquid whereby said developing liquid isdragged between said drum and said developing electrode due to therotation of said drum to thereby effect said develp g a 1 g 11.Apparatus as in claim 3 including a container for said developing liquidand an electrically conductive liquid having a larger specific gravitythan and immiscible with said developing liquid, said electricallyconductive. liquid acting as said developing electrode and where meansfor effecting said relative movement between said electrophotographicsheet and said developing electrode comprises a rotatably driven drumhaving development of said latent image and an inclined elec?trophotographic material'transferring mechanism, said developingelectrode arranged with a small space to the surface of theelectrophotographic material and a noz- I zle arranged about the top ofthe developing electrode for supplying a liquid developer between thedeveloping electrode and the electrophotographic material.

l t l l k

1. Apparatus for developing an electrostatic latent image on anelectrophotographic sheet of material optionally having anelectroconductive backing layer, said apparatus comprising: a developingelectrode disposed adjacent to and spaced from said electrophotographicsheet of material; means for providing a developing liquid between saiddeveloping electrode and said electrophotographic sheet of material todevelop said electrostatic latent image; resistive means for connectingeither or both of said developing electrode and said electroconductivebacking layer to a reference potential, the resistance of saidresistance means being in the range of 106 to 1012 ohms, to preventexcessive background fogging and image streaking during development ofsaid latent image.
 2. Apparatus as in claim 1 wherein said referencepotential is 0 volts.
 3. Apparatus as in claim 1 including means foreffecting relative movement between said electrophotographic sheet ofmaterial and said developing electrode.
 4. Apparatus as in claim 3including a container for storing said developing liquid and where saidmeans for effecting relative movement between said electrophotographicsheet of material and said developing electrode includes an endless beltdriven around a pair of rollers, said electrophotographic sheet beingdisposed on said belt for movement with respect to said developingelectrode and said endless belt being inclined by an angle of about 5*to 20* with respect to the horizontal plane and where said means forproviding said developing liquid between said electrophotographic sheetand said developing electrode is disposed adjacent the upper end of saidendless belt to introduce said developing liquid between saidelectrophotographic sheet and said developing electrode so that it flowstherebetween and then to said container for said developing liquid whichis disposed beneath the lower end of said endless belt.
 5. Apparatus asin claim 3 including a container for said developing liquid, saiddeveloping electrode being disposed in said developing liquid and wheresaid means for effecting said relative movement between saidelectrophotographic sheet and said developing electrode includes arotatably driven Drum having disposed around a portion thereof saidelectrophotographic sheet, said portion being partially disposed in saiddeveloping liquid and adjacent said developing electrode.
 6. Apparatusas in claim 3 including a first plurality of drive rollers disposed atone side of said electrophotographic sheet and where said developingelectrode includes a second plurality of drive rollers respectivelydisposed adjacent said first plurality of drive rollers at the otherside of said electrophotographic sheet, said first and second pluralityof drive rollers moving said electrophotographic sheet therebetween. 7.Apparatus as in claim 6 including a shaft for each of said secondplurality of rollers and a bearing for each of said shafts.
 8. Apparatusas in claim 7 where said resistive means is disposed on the sides ofsaid bearing and in electrical connection with said shaft and saidbearing being connected to said reference potential.
 9. Apparatus as inclaim 7 where at least the outer portion of said shaft comprises saidresistance means, said shaft being in electric contact with said bearingand said bearing being maintained at said reference potential. 10.Apparatus as in claim 3 including a container for said developing liquidand for a second liquid having a larger specific gravity than andimmiscible with said developing liquid and where said developingelectrode is disposed in said second liquid and where said means foreffecting relative movement between said electrophotographic sheet andsaid developing electrode comprises a rotatably driven drum havingdisposed around a portion thereof said electrophotographic sheet, saidportion being disposed completely in said developing liquid andpartially in said second liquid whereby said developing liquid isdragged between said drum and said developing electrode due to therotation of said drum to thereby effect said developing.
 11. Apparatusas in claim 3 including a container for said developing liquid and anelectrically conductive liquid having a larger specific gravity than andimmiscible with said developing liquid, said electrically conductiveliquid acting as said developing electrode and where means for effectingsaid relative movement between said electrophotographic sheet and saiddeveloping electrode comprises a rotatably driven drum having disposedaround a portion thereof said electrophotographic sheet, said portionbeing completely disposed in said developing liquid and partiallydisposed in said electrically conductive liquid.
 12. An apparatus fordeveloping electrostatic latent image on an electrophotographic materialcomprising a developing electrode connected to ground potential via aresistance in the range 106 -1012 Omega to prevent excessive backgroundfogging and image streaking during development of said latent image andan inclined electrophotographic material transferring mechanism, saiddeveloping electrode arranged with a small space to the surface of theelectrophotographic material and a nozzle arranged about the top of thedeveloping electrode for supplying a liquid developer between thedeveloping electrode and the electrophotographic material.